Semi-conductors such as mercury cadmium telluride used in the manufacture of I.R. detection devices require surface passivation in order to avoid surface contamination and in order to improve the performance of the I.R. detector. Various attempts have been made to improve the performance of such detectors, and amongst these there may be mentioned passivation by means of deposition of a surface film, such as a film of zinc sulfide; or by means of anodic oxidation of the surface of the semi-conductor.
The main drawbacks of semi-conductors treated as above is the poor thermal stability of the semi-conductors, which is required under varying conditions, such as vacuum packaging.
Semi-conductors passivated according to the prior art, which were used in I.R. detectors resulted in a wide variance in the performance of such devices. Anodic oxides produced on the surface of mercury-cadmium telluride contain a rather large fixed positive charge, which is typically of the order of 10.sup.12 cm.sup.-2, and this further increases upon thermal treatment at temperatures in the 60.degree. C. to 100.degree. C. range.
It is apparent that anodic oxides cause a strong accumulation of n-type mercury-cadium telluride (HgCdTe, for brief), and a strong inversion on p-type HgCdTe. The strong accumulation of n-type photoconductors lowers the resistance of the device and thus reduces voltage responsitivity of such devices. The inversion layer which is formed on the surface of p-type photovoltaic devices diminishes performance to essentially zero due to the formation of a conducting channel between adjacent diodes.